surfaces/adapter/websocket-api: support COPY TO command

[morsapaes]

Feature request

COPY TO isn't currently supported in the WebSocket API, which prevents users from running the new variation of the command introduced in #7256 (aka s3 bulk exports) via the SQL Shell. This is a pre-requisite to get s3 bulk exports into Private Preview, since otherwise users would need to spin up a separate SQL client like psql to run the new command.

-- SQL Shell
COPY mv TO 's3://mz-to-snow/'
WITH (
    AWS CONNECTION = aws_role_assumption,
    FORMAT = 'csv'
  );

Error: unsupported via this API: COPY (SELECT * FROM mv) TO 's3://mz-to-snow/' WITH (AWS CONNECTION = aws_role_assumption, FORMAT = 'csv')

From a quick chat with @maddyblue, it's possible to support this command variation (just not COPY TO stdout).